Method and apparatus for gas concentration detection and manufacturing method of the apparatus

ABSTRACT

An A/F sensor generates a current signal corresponding to an air-fuel ratio in response to a voltage applied by a bias control circuit. After a sensor current is received as a voltage signal via a voltage follower, it is outputted to an A/D converter having a predetermined input voltage range, 0 to 5V. A sensor current detection circuit has a plurality of current detection resistors. In order to variably set the resistance value by the sensor current detection circuit, a switch circuit is switched in accordance with the sensor current depending on whether the A/F value to be detected is in the zone near the stoichiometric ratio or in other air-fuel ratio zones.

RELATED APPLICATION

This application is a division of our prior application Ser. No.09/064,155 filed Apr. 22, 1998, now issued as U.S. Pat. No. 5,980,710.

CROSS REFERENCE TO RELATED APPLICATION

This application relates to and incorporates herein by referenceJapanese Patent Applications No. 9-131366 filed on May 21, 1997 and No.10-55149 filed on Mar. 6, 1998.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gas concentration detecting apparatusand method and a manufacturing method for the apparatus. The apparatusand method uses a gas concentration sensor for outputting a currentsignal corresponding to a gas concentration of gas to be detected when avoltage is applied. The apparatus and method is, for example, embodiedin a gas concentration detecting apparatus applied to a gasconcentration feedback control system of an engine employed in avehicle.

2. Related Art

For application to a vehicle, a gas concentration detecting apparatususing a gas concentration sensor is proposed. As one example, anair-fuel ratio detecting apparatus using an air-fuel ratio sensor isknown.

In an air-fuel ratio control of an engine mounted on a vehicle in recentyears, for example, there is a demand for improved control accuracy anda demand for a transition to lean-burn. In order to respond to thesedemands, a linear air-fuel ratio sensor for detecting the air-fuel ratioof air-fuel mixture supplied to the engine (concentration of oxygen inexhausted gas) linearly over a wide zone and an air-fuel ratio detectingapparatus using the sensor are implemented. As such an air-fuel ratiosensor, for example, in an air-fuel ratio sensor of a limit currenttype, the zone for detecting a limit current is shifted in accordancewith the air-fuel ratio (concentration of oxygen) at that time as isgenerally known.

The air-fuel ratio sensor of a limit current type has outputcharacteristics in which the farther the air-fuel ratio moves to thelean zone, the more the zone for detecting a limit-current is shifted tothe positive-voltage side. The farther the air-fuel ratio moves to therich side, the more the zone for detecting a limit current is shifted tothe negative-voltage side. Consequently, if the applied voltage is heldset at a fixed value when the air-fuel ratio changes, it would beimpossible to detect an air-fuel ratio accurately by using the zone fordetecting a limit current. In a conventional air-fuel ratio detectingapparatus, therefore, the voltage applied to the sensor is varied inaccordance with the air-fuel ratio at each time, that is, the sensorcurrent (for example, Japanese Patent Laid-Open Nos. Sho-61-237047 andSho-61-280560). In this case, the applied voltage is controlled on thebasis of an application voltage characteristic line Lx in FIG. 3. Bycontrolling the applied voltage in this way, a desired sensor current(limit current) can be always detected.

The circuit construction of an air-fuel ratio detecting apparatus whichis conventionally, implemented is generally shown in FIG. 22. In thediagram, a reference voltage Va generated by a reference voltage circuit84 is applied to one terminal 82 of an air-fuel ratio sensor 81 and aninstruction voltage Vb outputted from a D/A converter 87 is applied tothe other terminal 83. The instruction voltage Vb is variably controlledby a CPU (not shown) in accordance with an air-fuel ratio at each time.The circuit construction will be briefly described. The predeterminedreference voltage Va generated by the reference ID voltage circuit 84 isamplified by an amplification circuit 85. The same voltage Va as thereference voltage Va from the reference voltage circuit 84 is applied toone terminal 82 of the air-fuel ratio sensor 81. The instruction voltageVb outputted form the D/A converter 87 is amplified by an amplificationcircuit 86. The same voltage Vb as the instruction voltage Vb is appliedto the other terminal 83 of the air-fuel ratio sensor 81.

The linear type air-fuel ratio sensor 81 conducts a sensor currentaccording to the air-fuel ratio. An A/F output indicative of an air-fuelratio is therefore detected as an electromotive voltage Vc of a currentdetection resistor 88 for detecting the sensor current (air-fuel ratio),not a sensor terminal voltage as the predetermined reference voltage Va.In this case, the electromotive voltage Vc is outputted via a voltagefollower 89. FIG. 23 is a graph showing a characteristic of an outputvoltage (A/F value) of each air-fuel ratio. According to the diagram,when the air-fuel ratio is shifted to the lean side, the electromotivevoltage Vc is shifted to the positive side with respect to the referencevoltage Va. When the air-fuel ratio is shifted to the rich side, theelectromotive voltage Vc is shifted to the negative side with respect tothe reference voltage Va. A Vc signal (A/F value) obtained in thismanner is transmitted from the voltage follower 89 to an A/D converter90. After A/D-converted by the A/D converter 90, the resultant signal isused for the air-fuel ratio F/B control in a CPU 91 for engine control.

In the air-fuel ratio detecting apparatus having the aboveconfiguration, the input voltage range of the A/D converter 90 forreceiving the voltage signal (A/F value) is limited to a predeterminedrange of, for example, “0 to 5V”. For instance, in case of using an8-bit A/D converter, the input voltage range of “0 to 5V” is dividedinto 256 and the A/F value is read. Specifically, when the air-fuelratio detection range is set to a zone (A/F=12 to 18) near thestoichiometric ratio in order to perform a stoichiometric control inwhich the stoichiometric ratio (A/F=14.7) is used as a target air-fuelratio, the electromotive voltage Vc is outputted in the range of “0 to5V” by using the current detection resistor 88 in FIG. 22. In thisinstance, the voltage value per unit A/F (every “1” of the interval ofA/F) is “0.833V” and the A/F value is divided into 42 per unit A/F anddetected.

On the contrary, for example, in a case where the air-fuel ratiodetection range is expanded to A/F=12 to 25 in order to realizelean-burn control, when the air-fuel ratio detecting range is kept setto a range of “0 to 5V”, the voltage value per unit A/F is “0.384V” andthe A/F value is detected by being divided into 19 per unit A/F. Thatis, it denotes that the detection accuracy of the air-fuel ratio at thetime of the lean-burn control is lower than the detection accuracy ofthe air-fuel ratio at the time of the stoichiometric control (the higherthe voltage value per unit A/F is, the higher the detection accuracy ofthe air-fuel ratio is). As a result, for example, in the air-fuel ratiocontrol system in which both of the stoichiometric control and thelean-burn control are executed, a problem is caused such that thedetection accuracy of the air-fuel ratio near the stoichiometric ratiodeteriorates in order to assure the detection accuracy of the air-fuelratio at the time of lean-burn control.

It is to be noted that the problem of degradation in detection accuracyof the sensor occurs not only in the air-fuel ratio detecting apparatusbut also in all of gas concentration detecting apparatuses which use agas concentration sensor for producing a current signal in accordancewith the gas concentration to be detected and are constructed to detectgas concentration from a detection result of the sensor.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a gas concentrationdetecting apparatus and method in which the detection accuracy of gasconcentration can be improved even when gas concentration detection overa wide range is required.

It is another object of the present invention to provide a manufacturingmethod for gas concentration detecting apparatus by which gasconcentration detection output characteristics can be adjusted.

According to one aspect of the present invention, the resistance valueof a current detection resistor is set variably so that gasconcentration can be always detected in a voltage range which isreadable by a signal processor. For example, The voltage range is “0 to5V”. At this time, it is made possible to assure the highest accuracywithin a limitation that the gas concentration is detected within thevoltage range. That is, in whatever zones a detected value (sensorcurrent) of a gas concentration sensor resides, the detection accuracycan be assured.

According to another aspect of the present invention, a currentdetection resistor is provided to produce a plurality of detectionsignals at different voltage levels, and one of the detection signals isselected in accordance with a current value of a gas concentrationsensor.

According to a further aspect of the present invention, a switchingcondition for switching a resistance value of a current detectionresistor is discriminated, and the resistance value is variably set inaccordance with a discrimination result of the switching condition.

According to a still further aspect of the present invention, an outputvoltage of a sensor current detection resistor is monitored, and theoutput voltage is adjusted by trimming a plurality of voltage dividingresistors which produce a reference voltage to be applied to a gasconcentration sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention will bemade more apparent from the following detailed description withreference to the accompanying drawings. In the drawings:

FIG. 1 is a circuit diagram showing an air-fuel ratio detectingapparatus used as a gas concentration detecting apparatus according to afirst embodiment of the present invention;

FIG. 2 is a cross sectional view illustrating a construction of an A/Fsensor used as a gas concentration sensor;

FIG. 3 is a graph showing a V-I characteristic of the A/F sensor;

FIG. 4 is a graph showing an output voltage characteristics of the A/Fsensor for each air-fuel ratio;

FIG. 5 is a graph showing a relation between a sensor current and acurrent detection resistor in the first embodiment;

FIG. 6 is a flowchart showing an air-fuel ratio detecting routine in thefirst embodiment;

FIG. 7 is a graph showing an error in the output voltage of the air-fuelratio detecting apparatus;

FIG. 8 is a circuit diagram showing an apparatus for adjusting theoutput voltage of the air-fuel ratio detecting apparatus;

FIG. 9 is a circuit diagram showing an air-fuel ratio detectingapparatus according to a second embodiment of the present invention;

FIG. 10 is a graph showing an output voltage characteristic for eachair-fuel ratio in the second embodiment;

FIG. 11 is a graph showing the relation between the sensor current andthe current detection resistor in the second embodiment;

FIG. 12 is a flowchart showing an air-fuel ratio detecting routine inthe second embodiment;

FIG. 13 is a circuit diagram showing an air-fuel ratio detectingapparatus according to a third embodiment;

FIG. 14 is a circuit diagram showing an air-fuel ratio detectingapparatus according to a fourth embodiment;

FIG. 15 is a circuit diagram showing an air-fuel ratio detectingapparatus according to a fifth embodiment;

FIG. 16 is a flowchart showing a part of an air-fuel ratio detectingroutine in the fifth embodiment;

FIG. 17 is a time chart showing operation of the fifth embodiment;

FIG. 18 is a flowchart showing a timer interrupt routine executed by anengine control ECU in a sixth embodiment;

FIG. 19 is a flowchart showing a part of an air-fuel ratio detectingroutine in the sixth embodiment;

FIG. 20 is a flowchart showing a sensor deterioration discriminationprocessing in a seventh embodiment;

FIG. 21 is a graph showing the relation between the sensor current andthe current detection resistor in the seventh embodiment;

FIG. 22 is a circuit diagram showing a conventional air-fuel ratiodetecting apparatus; and

FIG. 23 is a graph showing an output voltage characteristic for eachair-fuel ratio in the conventional apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will be described hereinbelow with reference tovarious embodiments which are applied to air-fuel ratio detection in anengine control system.

(First Embodiment)

An air-fuel ratio detecting apparatus in the first embodiment is appliedto an air-fuel ratio feedback (F/B) control system of an electronicallycontrolled gasoline injection engine mounted on a vehicle and detectsthe air-fuel ratio on the basis of components of an exhaust gasexhausted from the engine. An engine control ECU 40 for controlling anair-fuel ratio F/B control selectively executes a stoichiometric controlin which the stoichiometric ratio (A/F=14.7) is a target air-fuel ratioand a lean-burn control in which a predetermined lean air-fuel ratio(for example, A/F=22) in a lean zone is a target air-fuel ratio inaccordance with an engine operating state.

In the apparatus according to this embodiment, a limit-current typeair-fuel ratio sensor (A/F sensor) 30 for outputting a current signal(limit current Ip) corresponding to the air-fuel ratio accompanyingapplication of a voltage Va-Vb is used and the voltage Vp applied to thesensor is controlled by a bias control circuit 10. The limit current Ipdetected by the A/F sensor 30 is extracted as a voltage signal and,A/D-converted by an A/D converter (signal processor) 41 having apredetermined input voltage range (0 to 5V in the embodiment), and afterthat, the resultant data is outputted to a CPU 42 in the engine controlECU. Especially, the apparatus according to this embodiment has aconstruction such that a resistance value of a current detectionresistor 15 provided in the bias control circuit 10 is variably set inorder to detect the air-fuel ratio with high accuracy in any air-fuelratio zone. Specifically, the resistance value of the current detectionresistor 15 is properly changed according to the zone near thestoichiometric ratio and other air-fuel ratio zones.

Referring to FIG. 2, the A/F sensor 30 is installed so as to protrudetoward the inside of an engine exhaust pipe 39. Major components of thesensor 30 are a cover 31, a sensor body 32, and a heater 33. The cover31 has a U-character shape in cross section and a number of small holes31 a are bored through the peripheral wall of the cover 31. The sensorbody 32 generates a limit current corresponding to theoxygen-concentration in the lean zone of an air-fuel ratio or theconcentration of unburned gas (such as CO, HC, and H₂) in the rich zoneof the air-fuel ratio.

In the sensor body 32, an exhaust-side electrode layer 36 is firmlyattached to the external surface of a solid electrolyte layer 34 formedin a cup shape in cross section and an atmosphere-side electrode layer37 is firmly attached to the internal surface of the solid electrolytelayer 34. On the outer side of the exhaust-side electrode layer 36, adiffusion resistance layer 35 is formed by a plasma spraying method orthe like. The solid electrolyte layer 34 is made of an oxygen ionconducting oxide sintered body which is solid-solved in a material suchas ZrO₂, HfO₂, ThO₂, and Bi₂O₃ with a material such as CaO, MgO, Y₂O₃,and Yb₂O₃ used as a stabilizer. The diffusion resistance layer 35 ismade of a heat resisting inorganic material such as alumina, magnesia,silica, spinel and mullite. The exhaust-side electrode layer 36 and theatmosphere-side electrode layer 37 are both made of a noble metal with ahigh catalytic activity such as platinum and have the surfaces to whicha porous chemical plating is performed. The area and the thickness ofthe exhaust-side electrode layer 36 is 10 to 100 mm² and about 0.5 to2.0 μm, respectively. On the other hand, the area and the thickness ofthe atmosphere-side electrode layer 37 are 10 mm² or larger and about0.5 to 2.0 μm.

The heater 33 is housed in the internal space formed by theatmosphere-side electrode layer 37 and heats the sensor body 32(atmosphere-side electrode layer 37, the solid electrolyte layer 34, theexhaust-side electrode layer 36, and the diffusion resistance layer 35)by its heat generating energy. The heater 33 has a heat generatingcapacity sufficient to activate the sensor body 32.

In the A/F sensor 30 having the configuration described above, thesensor body 32 generates a limit current according to the concentrationof oxygen in a zone leaner than the stoichiometric ratio point(stoichiometric air-fuel ratio point). In this case, the limit currentcorresponding to the concentration of oxygen is determined by the areaof the exhaust-side electrode layer 36, and the thickness, the porosityand the average pore diameter of the diffusion resistance layer 35. Thesensor body 32 is capable of detecting the concentration of oxygen inaccordance with a linear characteristic thereof. It is thereforenecessary to hold the element temperature at a high temperature equal toor higher than about 600° C. in order to activate the sensor body 32. Ina zone richer than the stoichiometric ratio, the concentration ofunburned gases such as carbon monoxide (CO) changes almost linearly withthe air-fuel ratio and the sensor body 32 generates a limit currentaccording to the concentration of CO or the like.

It will be understood from FIG. 3 that current flowing in the solidelectrolyte layer 34 of the sensor body 32, is proportional to the A/Fdetected by the A/F sensor 30 and changes linearly with respect to avoltage applied to the solid electrolyte layer 34. In this case,straight line segments parallel to the voltage axis V constitute a limitcurrent limited detection zone which specifies the limit current of thesensor body 32. Increases and decreases of the limit current (sensorcurrent) correspond to increases and decreases in the A/F (that is, thedegree of lean and rich). That is, the more the A/F is shifted to thelean side, the more the limit current increases. The more the A/F isshifted to the rich side, the more the limit current decreases.

In the V-I characteristic, a voltage zone below the straight linesegments (limit current detection zone) parallel to the voltage axis Vis a resistance dominated zone. The gradient of the linear straight linesegments in the resistance dominated zone is specified by the internalresistance (element resistance) of the solid electrolyte layer 34 in thesensor body 32. Since the element resistance changes with change intemperature, when the temperature of the sensor body 32 decreases, thegradient is reduced by the increase in the element resistance.

In the V-I characteristic of FIG. 3, a “sensor current detection range”is set between an extreme rich zone and an extreme lean zone and a“dynamic range” as an air-fuel ratio detection range is set within thesensor current detection range. According to the air-fuel ratio controlsystem of this embodiment, since a lean-burn control is performed, thedynamic range is set in a range of A/F=12 to 25.

Referring back to FIG. 1, the bias control circuit 10 is a circuit forcontrolling a voltage applied to the A/F sensor 30 and has the followingconfiguration. That is, the bias control circuit 10 has a referencevoltage circuit 11. The reference voltage circuit 11 generates apredetermined reference voltage Va (2.5V in the embodiment) by dividinga constant voltage Vcc by voltage dividing resistors 12 and 13.

A voltage dividing point of the reference voltage circuit 11 at whichthe reference voltage Va exists is connected to a non-inversion inputterminal of an amplifier 14 a in an amplification circuit 14. Oneterminal 25 of the A/F sensor 30 is connected to the output terminal ofthe amplifier 14 a via a sensor current detection circuit 15. Theterminal 25 is a terminal connected to the atmosphere side electrodelayer 37 in the A/F sensor 30. The same voltage Va (2.5V) as thereference voltage Va of the reference voltage circuit 11 is alwaysapplied to the terminal 25. The terminal 25 is connected to an inversioninput terminal of the amplifier 14 a and the voltage Va of the terminal25 is received by the A/D converter 22.

The sensor current detection circuit 15 detects a sensor current Ipaccording to the air-fuel ratio at each time and has two currentdetection resistors 15 a and 15 b which are serially connected betweenthe output terminal of the amplifier 14 a and the terminal 25 of the A/Fsensor 30. A voltage Vc at a connecting point (C point in the diagram)of the current detection resistors 15 a and 15 b is received by the A/Dconverter 22.

A CPU 21 for bias control receives voltages from both ends of thecurrent detection resistor 15 a through the A/D converter 22 and detectsthe sensor current (limit current) Ip at that time from theA/D-converted data of the both-end voltages (Va and Vc). The CPU 21computes an instruction value of a voltage for applying to the A/Fsensor 30 in accordance with the sensor current Ip at that time.Specifically, an application voltage linear line Lx shown in FIG. 3 isused and an application voltage according to the sensor current Ip atthat time is determined. The voltage instruction value calculated by theCPU 21 is converted to an instruction voltage Vb by a D/A converter 23and the instruction voltage Vb after the D/A conversion is applied to anamplification circuit 16.

The D/A converter 23 is connected to the non-inversion input terminal ofan amplifier 16 a in the amplification circuit 16. An inversion inputterminal of the amplifier 16 and the other terminal 26 of the A/F sensor30 are connected to the output terminal of the amplifier 16 a. In thiscase, the terminal 26 is a terminal connected to the exhaust-sideelectrode layer 36 of the A/F sensor 30 and the same voltage Vb as theinstruction voltage Vb as an output of the D/A converter 23 is appliedto the terminal 26.

At the time of the air-fuel ratio detection, therefore, in the biascontrol circuit 10 having the above construction, the reference voltageVa is always supplied to the terminal 25 which is one of the terminalsof the A/F sensor 30 and the instruction voltage Vb is applied to theother terminal 26. When the instruction voltage Vb supplied to the otherterminal 26 of the A/F sensor 30 via the D/A converter 23 is lower thanthe reference voltage Va (if Vb<Va), a positive bias is applied to theA/F sensor 30. If the instruction voltage Vb supplied to the otherterminal 26 of the A/F sensor 30 is higher than the reference voltage Va(if Vb>Va), a negative bias is applied to the A/F sensor 30. In eithercase, the sensor current Ip which flows with the application of voltageis detected as a difference (Vc−Va) between the electric potentials ofthe ends of the current detection resistor 15 a and is supplied to theCPU 21 by way of the A/D converter 22.

In addition, the bias control circuit 10 has a voltage follower 17 forreceiving the sensor current Ip flowing the current detection circuit 15as a voltage signal and outputting the received voltage signal to anengine control ECU 40 on the outside. The point C or the point D in thediagram is connected to the non-inversion input terminal of the voltagefollower 17 in accordance with the switched position of a switch circuit18. The point C is a connecting point of the current detection resistors15 a and 15 b and the point D is a connecting point of the outputterminal of the amplifier 14 a and the current detection resistor 15 b.

In this case, when the switch circuit 18 is turned to the voltage Vcside as shown in the diagram, the voltage Vc at the point C is used as avoltage Vf at the non-inversion input terminal of the voltage follower17. That is, the sensor current Ip flowing through the current detectionresistors 15 a and 15 b is detected only by the resistance of thecurrent detection resistor 15 a and the voltage Vc corresponding to Ipis supplied to the voltage follower 17 via the switch circuit 18.

When the switch circuit 18 is changed from the position shown in thediagram to the voltage Vd side, the voltage Vd at the point D is used asthe voltage Vf at the non-inversion input terminal of the voltagefollower 17. That is, the sensor current Ip is detected by theresistance of the current detection resistors 15 a and 15 b and thevoltage Vd corresponding to Ip is supplied to the voltage follower 17via the switch circuit 18. The switching operation of the switch circuit18 is performed by the CPU 21.

The voltage output of the voltage follower 17 is inputted to a CPU 42via an A/D converter 41 in the engine control ECU 40. The CPU 42 detectsan actual air-fuel ratio on the basis of the difference between the A/Fvalue (voltage value) inputted via the A/D converter 41 and thereference voltage Va of the bias control circuit 10. In the A/Dconverter 41 of this embodiment, the power source voltage is a constantvoltage Vcc of “5V” (not shown in the diagram) and the input voltagerange which can be read by the A/D converter 41 is set to “0 to 5V”. Inthis case, if the 8-bit A/D converter 41 is used, the input voltagerange of “0 to 5V” is divided into 256 to read the A/F values.

With respect to the air-fuel ratio F/B control by the engine control ECU40, since it is not the gist of the case and the control is known, itsdetailed description is omitted here. The engine control ECU 40 receivesthe detection result (voltage signal) of the air-fuel ratio by the A/Fsensor 30 and F/B controls the air-fuel ratio in accordance with acontrol algorithm such as the advanced control or a PID control on thebasis of the detection result. The engine control ECU 40 controls theamount of fuel injected from an injector (not shown) to each ofcylinders of the engine so that the air-fuel ratio at each timecoincides with the target air-fuel ratio. In this instance, if theengine is in a low load state, lean-burn control is performed and if theengine is in an intermediate or high load state, an ordinarystoichiometric control is executed.

The switching operation of the switch circuit 18 will be described byshowing actual specific values. Methods of detecting the air-fuel ratiowith respect to the following two zones will be described here;

A zone (A/F=12.8 to 18) near the stoichiometric ratio in the dynamicrange

Other air-fuel ratio zones (A/F=12 to 12.8, 18 to 25) In the apparatusaccording to this embodiment, the zone near the stoichiometric ratiowhere A/F=12.8 to 18 corresponds to the air-fuel ratio detection rangewhich is necessary at the time of the stoichiometric control and theair-fuel ratio zone where A/F=18 to 25 corresponds to the air-fuel ratiodetection range which is necessary at the time of lean-burn control.

The reference voltage Va is set to “2.5V”, the sensor current Ip whenA/F=18 is set to “7 mA”, and the sensor current Ip when A/F=25 is set to“22 mA” (V-I characteristic of FIG. 3). A resistance value R1 of thecurrent detection resistor 15 a is set to “113 Ω” and a resistance valueR2 of the current detection resistor 15 b is set to “224 Ω”.

First, in the zone (A/F=12.8 to 18) near the stoichiometric ratio, theair-fuel ratio at which the voltages Vc and Vd at the points C and D inFIG. 1 are maximum is A/F=18. The voltages Vc and Vd when A/F=18 are

Vc=3.291V, and

 Vd=4.999V.

In connection, the voltage Vc is obtained by adding the referencevoltage Va to the product of the sensor current Ip and the resistancevalue R1 of the current detection resistor 15 a (Vc=Ip·R1+Va). Thevoltage Vd is obtained by adding the reference voltage Va to the productof the sensor current Ip and the resistance values R1+R2 of the currentdetection resistors 15 a and 15 b (Vd=Ip·(R1+R2)+Va).

Since both of the values of the voltages Vc and Vd are within thevoltage range (0 to 5V) which can be dealt by the A/D converter 41 inthe engine control ECU 40, both of the values can be read by the A/Dconverter 41. In order to assure the detection accuracy of the air-fuelratio as described above, it is preferable to set the voltage value perunit A/F as large as possible.

When the voltage values of the voltages Vc and Vd per unit A/F arecalculated by using the stoichiometric ratio (A/F=14.7) as a reference,the voltage value per unit A/F of the voltage Vc is obtained as “0.239V”from the following arithmetic expression.

(3.291V−2.5V)/(18−14.7)

The voltage value per unit A/F of the voltage Vd is obtained as “0.757V”from the following arithmetic expression.

(4.999V−2.5V)/(18−14.7)

In this case, the fact that the latter has the larger voltage value perunit A/F denotes the voltage Vd has higher detection accuracy than thevoltage Vc. There is a similar tendency for any air-fuel ratio if it iswithin the zone (A/F=12.8 to 18) near the stoichiometric ratio. That is,in the zone near the stoichiometric ratio, by using the Vd value as theinput voltage Vf of the voltage follower 17, the detection accuracy ofthe air-fuel ratio can be assured.

This output voltage characteristic will be described with reference toFIG. 4. If the voltage value per unit A/F when A/F=18 of a case wherethe value of the current detection resistor is “R1” (in case ofoutputting the voltage Vc) is compared with that of a case where thevalue of the current detection resistor is “R1+R2” (in case ofoutputting the voltage Vd), it will be understood that the latter one islarger and the detection accuracy of the air-fuel ratio is improved.

On the other hand, in the air-fuel ratio zones (A/F=12 to 12.8, 18 to25) other than the zone near the stoichiometric ratio, the air-fuelratio at which the voltages Vc and Vd at the points C and D in FIG. 1are maximum is A/F=25. The voltages Vc and Vd when A/F=25 are asfollows.

Vc=4.986V

Vd=10.354V

(Vc=Ip·R1+Va, Vd=Ip·(R1+R2)+Va)

In this case, since the input voltage range of the A/D converter 41 is“0 to 5V”, although the voltage Vc can be read, the voltage Vd cannot beread. In the air-fuel ratio zones (A/F=12 to 12.8, 18 to 25) other thanthe zone near the stoichiometric ratio, the Vc value is used as theinput voltage Vf of the voltage follower 17. That is, as shown in theoutput voltage characteristic of FIG. 4, the value of the currentdetection resistor has to be set to “R1” (value of the current detectionresistor 15 a). Thus, the air-fuel ratio of a maximum is detected.

FIG. 5 is a graph showing a preferable relation between the sensorcurrent Ip (mA) and the resistance (Ω) of the current detection resistoraccording to the Ip value. In the diagram, Ip=−11 mA when A/F=12, Ip=−7mA when A/F=12.8, Ip=7 mA when A/F=18, and Ip=22 mA when A/F=25.According to the diagram, it is understood that it is sufficient that

the current detection resistance is set to “357 Ω” corresponding to thevalue of “R1+R2” when −7 mA≦Ip≦7 mA (when A/F=12.8 to 18), and

the current detection resistance is set to “113 Ω” corresponding to thevalue of “R1” in the case where −11 mA≦Ip<−7 mA and 7 mA<Ip≦22 mA (incase of A/F=12 to 12.8, 18 to 25).

The operation of the air-fuel ratio detecting apparatus constructed asmentioned above will be described. FIG. 6 is a flowchart showing anair-fuel ratio detecting routine executed by the CPU 21. The CPU 21repeatedly executes the routine in a predetermined cycle (for example,at intervals of 4 ms).

The CPU 21 detects the sensor current Ip flowing according to theair-fuel ratio at each time in steps 101 to 103. In detail, the CPU 21reads one terminal voltage Va of the current detection resistor 15 a viathe A/D converter 22 in step 101. In the following step 102, the otherterminal voltage Vc of the current detection resistor 15 a is read viathe A/D converter 22. After that, in step 103, the CPU 21 calculates thepresent sensor current Ip on the basis of the voltages Va and Vc readthrough the A/D converter 22 by using the operational equation

Ip=(Vc−Va)/R1

(where, R1 is a resistance value of the current detection resistor 15a).

Then, the CPU 21 obtains a target application voltage corresponding tothe calculated sensor current Ip by using the application voltagecharacteristic line Lx shown in FIG. 3 in step 104 (map calculation).Further, in step 105, the CPU 21 applies the obtained target applicationvoltage as a voltage instruction value (instruction voltage Vb) via theD/A converter 23 to the A/F sensor 30.

The CPU 21 discriminates whether the sensor current Ip at that time lieswithin a range of “−7 mA to 7 mA” or not in step 106. Ip=−7 mA, 7 mA arethresholds used to discriminate whether the air-fuel ratio at that timeis in the zone near the stoichiometric ratio (A/F=12.8 to 18) or not. Ifthe step 106 is affirmatively discriminated, it denotes that theair-fuel ratio at that time lies within the zone near the stoichiometricratio.

When the step 106 is affirmatively discriminated (in case of −7 mA≦Ip≦7mA), the CPU 21 connects the switch circuit 18 to the voltage Vd side instep 107. Consequently, the voltage Vd serves as the input voltage Vf ofthe voltage follower 17 and the Vd value is outputted as an A/F outputto the A/D converter 41 in the engine control ECU 40. In this instance,the A/F output detected by the sensor current detection circuit 15 isdetected by the sum “R1+R2” of both of the resistance values of thecurrent detection resistors 15 a and 15 b.

When the step 106 is negatively discriminated, the CPU 21 allows theswitch circuit 18 to be connected to the voltage Vc side in step 108.Consequently, the voltage Vc serves as the input voltage Vf of thevoltage follower 17 and the Vc value is ID outputted as an A/F output tothe A/D converter 41 in the engine control ECU 40. In this instance, theA/F output detected by the sensor current detection circuit 15 isdetected by the resistance value “R1” of the resistor 15 a which is oneof the current detection resistors.

On the other hand, in the air-fuel ratio detecting apparatus accordingto this embodiment, the output value is adjusted by the followingprocedures in the manufacturing process of the apparatus to eliminateindividual variations from apparatus to apparatus. The air-fuel ratiooutput signal (output signal of the voltage follower 17) of the biascontrol circuit 10 causes detection errors for the following reasons.

(1) variations of the resistors 12 and 13 of the reference voltagecircuit 11,

(2) offset error of the operational amplifiers 14 a and 17,

(3) variation of the sensor current detection circuit 15, and the like.

The procedure for adjusting the output voltage is explained withreference to FIGS. 7 and 8. Here, the reference voltage Va of the A/Fsensor 30 determined by the voltage dividing resistors 12 and 13 in thereference voltage circuit 11 is “2.5V”, the resistance value of thesensor current detection circuit 15 is “357 Ω”, the sensor current atA/F=18 is “7 mA”, the sensor current at A/F=17 is “4.880 mA”. In thisinstance, the output voltage at A/F=18 is:

357 Ω·7 mA+2.5V=4.999V

Further, the output voltage at A/F=17 is:

357 Ω·4.880 mA+2.5V=4.242V

Therefore, the width of the voltge for the unit A/F (1 A/F) is 0.757V(=4.999−4.242).

To be more specific with regard to the above variations (1)-(3), becausethe voltage dividing resistors 12 and 13 have variations of about ±1%,the reference voltage Va varies within a range of 1% (that is, ±25 mV)of 2. (above (1)).

Further, because the operational amplifiers 14 a and 17 have the offsetvoltage Voff of about ±20 mV, an error occurs between the positive (+)side and negative (−) side terminls in each of the operationalamplifiers 14 a and 17 (abobe variations (2)). The sum of the errors(offset voltage Voff) of the two operational amplifiers 14 a and 17amounts to about ±40 mV.

Still further, in the sensor current detection circuit 15, the currentdetection resistors 15 a and 15 b have variations of about ±1%. As aresult, voltage error corresponding to the variations of the sensorcurrent and the current detection resistors 15 a and 15 b occurs atother than the stoichiometric ratio where the sensor current becomesOmA. For instance, the error at A/F=17 is:

4.880 mA·357 Ω·0.01=17.4 mV (above (3)).

The maximum sum of the errors of the above (1) to (3) at A/F=17 is:

25 mV+40 mV+17.4 mV=82.4 mV

That is, as shown in FIG. 7, the relation between the air-fuel ratio andthe output voltage includes variations shown by the two-dot chain lineagainst an ideal characteristics shown by the solid line due to theabove reasons (1) to (3). At A/F=17, the error becomes ±82.4 mV atmaximum against the ideal output voltage. This output voltage errorcorresponds to A/F error of:

82.4 mv/0.757V=0.11

According to the present embodiment, therefore, the voltage dividingresistors 12 and 13 of the reference voltage circuit 11 in the biascontrol circuit 10 are trimmed appropriately to obviate individualvatiation of the device caused by the above errors. Though the circuitshown in FIG. 8 has the same construction as the bias control circuit 10shown in FIG. 1, its construction such as the sensor current detectioncircuit 15 is shown in a simplified form partly for brevity.

Specifically, a constant current source 101 is connected to the terminal100 of the bias control circuit 10 to provide a current of a constantvalue by the constant current source. For instance, with a current of“4.880 mA” which corresponds to A/F=17 being provided, the outputvoltage of the bias control circuit 10 (potential at Z-point in FIG. 8)is measured. at this time, the voltage dividing resistor 12 or 13 istrimmed in accordance with the deviation of the output voltage at theZ-point from the ideal vlaue “4.242V” of the output voltage at A/F=17.

If the output voltage measured at the Z-point is higher than the idealvalue “4.242V”, the voltage dividing resistor 12 is trimmed. By trimmimgthe voltage dividing resistor 12 to a larger resistance, the referencevoltage Va produced from the reference voltage circuit 11 is decreased.As the output voltage at the Z-point decreases in proportion to thereference voltage Va, the output voltage of the Z-point is made closerto the ideal value by trimming the voltage dividing resistor 12. Thisprocedure enables provision of the ouput voltage which has the leastvariation from the ideal value.

If the output voltage measured at the Z-point is lower than the idealvalue “4.242V” at A/F=17, the voltage dividing resistor 13 is trimmed.By trimmimg the voltage dividing resistor 13 to a larger resistance, thereference voltage Va produced from the reference voltage circuit 11 isincreased. Thus, the voltage of the Z-point is made closer to the idealvalue to provide the ouput voltage which has the least variation fromthe ideal value. The voltage dividing resistors 12 and 13 may be trimmedby thick film resistor trimming method, thin film resistor trimmingmethod which can be performed on a chip or the like trimming method.

According to the embodiment described in detail, the following effectscan be obtained.

(a) According to this embodiment, in the air-fuel ratio detectingapparatus for converting the sensor current Ip into a voltage signal andoutputting the voltage signal to the A/D converter 41 via the voltagefollower 17, the resistance value of the sensor current detectioncircuit 15 for sending the voltage signal to the voltage follower 17 isvariably set in accordance with the sensor current Ip. According to theconstruction, the air-fuel ratio can be always detected in the voltagerange which can be read by the A/D converter 41, that is, the voltagerange of “0 to 5V”. Further, a high detection accuracy can be assuredwithin a limitation that the air-fuel ratio should be detected in theabove voltage range. Consequently, the detection accuracy of theair-fuel ratio can be improved even when a wide air-fuel ratio detectionrange is required. As a result, also in the air-fuel ratio controlsystem in which both of stoichiometric control and lean-burn control areperformed, the detection accuracy of the air-fuel ratio near thestoichiometric ratio can be improved while assuring the detectionaccuracy of the air-fuel ratio at the time of the lean-burn control.

(b) According to embodiment, the current detection resistors 15 a and 15b are constructed by a plurality of resistors whose resistance valuesare known and the resistor connected to the input terminal of thevoltage follower 17 is properly changed in accordance with the sensorcurrent Ip. In this case, by switching the resistance value by theswitch circuit 18, the switching operation can be realized with a simpleconstruction.

(c) Further, the resistance value of the current detection resistor ischanged for each zone by dividing into a plurality of air-fuel ratiozones an (air-fuel ratio zone having a center at the target air-fuelratio) with respect to the target air-fuel ratio (A/F=14.7, 22) at timeof the stoichiometric control or lean-burn control as a reference. Thus,the detection accuracy as required can be assured at the air-fuel ratiopoint where air-fuel ratio detection accuracy is required.

(d) In an air-fuel ratio control system using air-fuel ratio detectingapparatus with the above construction, since the detection accuracy ofthe A/F value as a control parameter is enhanced, air-fuel ratio F/Bcontrol with high accuracy can be realized and excellent effects suchthat emission and fuel consumption is reduced can be obtained.

(e) The switch circuit 18 for switching the resistnce value of thesensor current detection circuit 15 is provided not in the sensorcurrent flow path but at the input side of the voltage follower 17. Inthis instance, the disadvantage that the air-fuel ratio detectionacuracy degrades due to variations in the current signal caused by aresistance component of the switch circuit 18 can be obviated.

(f) Further, at the time of manufacturing the air-fuel ratio detectingapparatus, the output voltage of the voltage follower 17 is monitoredand the voltage dividing resistors for the reference voltage are trimmedappropriately to adjust the output voltage. As a result, variations inoutput arising from the individual variation of the air-fuel ratiodetecting apparatus (bias control circuit 10) can be reduced andaccuracy in the air-fuel ratio detection can be enhanced to a higherlevel.

The second to sixth embodiments of the invention will now be described.In each of the following embodiments, portions equivalent to those inthe above-mentioned first embodiment are designated by the same numeralsand their descriptions are simplified. The points different from thefirst embodiment will be mainly described hereinbelow.

(Second embodiment)

The second embodiment of the invention will be described with referenceto FIGS. 9 to 10. FIG. 9 is a circuit diagram showing the outline of anair-fuel ratio detecting apparatus in the embodiment. Since theconstruction of the apparatus is basically similar to that of FIG. 1 ofthe first embodiment, only different points will be describedhereinbelow.

A sensor current detection circuit 15 has three current detectionresistors 15 a, 15 b, and 15 c which are serially connected between theoutput terminal of the amplifier 14 a and the terminal 25 of the A/Fsensor 30. The point C, D, or E in the diagram is connected to thenon-inversion input terminal of the voltage follower 17 which receivesthe sensor current Ip as a voltage signal in accordance with theswitching position of a switch circuit 52. The point C is a connectingpoint of the current detection resistors 15 a and 15 b. The point D is aconnecting point of the current detection resistors 15 b and 15 c. Thepoint E is a connecting point of the output terminal of the amplifier 14a and the current detection resistor 15 c.

In this case, when the switch circuit 52 is turned to the voltage Vcside as shown in the diagram, the voltage Vf of the input terminal ofthe voltage follower 17 becomes the voltage Vc at the point C. That is,the sensor current Ip flowing in the sensor current detection circuit 15is detected only by the resistance of the current detection resistor 15a. The voltage Vc corresponding to Ip is supplied to the voltagefollower 17 via the switch circuit 52.

When the switch circuit 52 is changed from the position shown in thediagram to the voltage Vd side, the voltage Vf of the input terminal ofthe voltage follower 17 becomes the voltage Vd at the point D. That is,the sensor current Ip is detected by the resistance of the currentdetection resistors 15 a and 15 b and the voltage Vd corresponding to Ipis supplied to the voltage follower 17 via the switch circuit 52.

Further, when the switch circuit 52 is changed from the position shownin the diagram to the voltage Ve side, the voltage Vf of the inputterminal of the voltage follower 17 becomes the voltage Ve at the pointE. That is, the sensor current Ip is detected by the resistance of thecurrent detection resistors 15 a, 15 b, and 15 c. The voltage Vecorresponding to this Ip is supplied to the voltage follower 17 via theswitch circuit 52.

The switching operation of the switch circuit 52 is controlled by theCPU 21 in a manner similar to the first embodiment. The switchingoperation of the switch circuit 52 will be described by showing actualspecific values. Methods of detecting the air-fuel ratio will bedescribed here with respect to each of the following three zones.

first air-fuel ratio zone (A/F=12.8 to 18) as a zone near thestoichiometric ratio in the dynamic range

second air-fuel ratio zone (A/F=12 to 12.8, 18 to 22) as zones outsideof the first air-fuel ratio zone

third air-fuel ratio zone (A/F=22 to 25) as a zone outside of the secondair-fuel ratio zone.

In the embodiment, the reference voltage Va is “2.5V”, the sensorcurrent Ip when A/F=18 is “7 mA”, the sensor current Ip when A/F=22 is“15.5 mA”, and the sensor current Ip when A/F=25 is “22 mA” (V-Icharacteristic of FIG. 3). A resistance value R11 of the currentdetection resistor 15 a is set to “113 Ω”, a resistance value R12 of thecurrent detection resistor 15 b is set to “148 Ω”, and a resistancevalue R13 of the current detection resistor 15 c is set to “196 Ω”.

In the first air-fuel ratio zone (A/F=12.8 to 18), the air-fuel ratio atwhich the voltages Vc, Vd, and Ve at the points C, D, and E in FIG. 9are maximum is A/F=18. The voltages Vc, Vd, and Ve when A/F=18 are asfollows.

Vc=3.291V

Vd=3.627V

Ve=4.999V

The voltage Vc is obtained by adding the reference voltage Va to theproduct of the sensor current Ip and the resistance value R11 of thecurrent detection resistor 15 a (Vc=Ip·R11+Va). The voltage Vd isobtained by adding the reference voltage Va to the product of the sensorcurrent Ip and the resistance values (R11+R12) of the current detectionresistors 15 a and 15 b (Vd=Ip·(R11+R12)+Va). The voltage Ve is obtainedby adding the reference voltage Va to the product of the sensor currentIp and the resistance values (R11 +R12+R13) of the current detectionresistors 15 a, 15 b, and 15 c (Ve=Ip·(R11+R12+R13)+Va).

Since all of the values of the voltages Vc, Vd, and Ve are within avoltage range (0 to 5V) which can be dealt by the A/D converter 41 inthe engine control ECU 40, each value can be read by the A/D converter41. As described before, however, in order to assure detection accuracyof the air-fuel ratio, it is desirable to increase the voltage value perunit A/F as much as possible.

When the voltage value per unit A/F of each of the voltages Vc, Vd, andVe is calculated by using the stoichiometric ratio (A/F=14.7) as areference, the voltage value per unit A/F of the voltage Vc is obtainedas “10.239V” from the following arithmetic expression.

(3.291V−2.5V)/(18−14.7)

The voltage value per unit A/F of the voltage Vd is obtained as “0.341V”from the following arithmetic expression.

(3.627V−2.5V)/(18−14.7)

The voltage value per unit A/F of the voltage Ve is obtained as “0.757V”from the following arithmetic expression.

(4.999V−2.5V)/(18−14.7)

In this case, since the voltage value per unit A/F of the voltage Ve isthe largest, it can be the that the voltage Ve has highest detectionaccuracy. There is a similar tendency for any A/F values if it is withinthe zone near the stoichiometric ratio (A/F=12.8 to 18). That is, in thezone near the stoichiometric ratio, the Ve value is used as the inputvoltage Vf of the voltage follower 17, thereby enabling the detectionaccuracy of the air-fuel ratio to be assured.

The output voltage characteristic will be described with reference toFIG. 10. If the voltage value per unit A/F when A/F=18 is compared withrespect to each of:

(a) a case where the value of the current detection resistance is set to“R11” (in case of outputting the voltage vc),

(b) a case where the value of the current detection resistance is set to“R11+R12” (in case of outputting the voltage Vd), and

(c) a case where the value of the current detection resistance is set to“R11+R12+R13” (in case of outputting the voltage Vd),

it will be understood that the value of (c) is the largest and thedetection accuracy of the air-fuel ratio is improved.

On the other hand, in the second air-fuel ratio zone (A/F=12 to 12.8, 18to 22), the air-fuel ratio at which the voltages Vc, Vd, and Ve at thepoints C, D, and E in FIG. 9 are maximum is A/F=22. The voltages Vc, Vd,and Ve when A/F=22 are as follows.

Vc=4.251V

Vd=4.996V

Ve=8.034V

In this case, since the input voltage range of the A/D converter 41 inthe engine control ECU 40 is “0 to 5V”, the voltages Vc and Vd excludingVe are readable values. When the voltages Vc and Vd are compared, theair-fuel ratio can be detected with higher accuracy by using the voltageVd (reason is similar to that when A/F=18). In the second air-fuel ratiozone (A/F=12 to 12.8, 18 to 22), therefore, the Vd value is used as theinput voltage Vf of the voltage follower 17. That is, as shown by theoutput voltage characteristic of FIG. 10, it is sufficient that thevalue of the current detection resistor is “R11+R12” (an addition valueof the current detection resistors 15 a and 15 b).

Further, in the third air-fuel ratio zone (A/F=22 to 25), the air-fuelratio at which the voltages Vc, Vd and Ve at the points C, D and E inFIG. 9 are maximum is A/F=25. The voltages Vc, Vd, and Ve when A/F=25are as follows.

Vc=4.986V

Vd=6.042V

Ve=10.354V

In this case, since the input voltage range of the A/D converter 41 is“0 to 5V”, although the voltage Vc can be read, the voltages Vd and Vecannot be read. In the third air-fuel ratio zone (A/F=22 to 25),therefore, the Vc value is used as the input voltage Vf of the voltagefollower 17. That is, as shown by the output voltage characteristic ofFIG. 10, it is sufficient that the value of the current detectionresistor is “R1” (value of the current detection resistor 15 a).

FIG. 11 is a graph showing a preferable relation between the sensorcurrent Ip (mA) and the resistance (Ω) of the current detection resistoraccording to the Ip value. In the diagram, Ip=−11 mA when A/F=12, Ip=−7mA when A/F=12.8, Ip=7 mA when A/F=18, Ip=15.5 mA when A/F=22, and Ip=22mA when A/F=25. According to the diagram, it will be understood that itis sufficient to set as follows.

In case of −7 mA≦Ip≦7 mA (when A/F=12.8 to 18), the current detectionresistance is set to “357 Ω” corresponding to the value of“R11+R12+R13”.

In case of −11 mA≦Ip<−7 mA, 7 mA<Ip≦1.5 mA (when A/F=12 to 12.8, 18 to22), the current detection resistance is set to “161 Ω” corresponding tothe value of “R11+R12”.

In case of 15.5 mA<Ip≦22 mA (when A/F=22 to 25), the current detectionresistance is set to “113 Ω” corresponding to the value of “R11”.

The operation of the air-fuel ratio detecting apparatus constructed asabove will be explained.

FIG. 12 is a flowchart showing an air-fuel ratio detecting routineexecuted by the CPU 21. The CPU 21 repeatedly executes this routine in apredetermined cycle (for example, an interval of 4 ms).

First, in steps 101 to 105, the CPU 21 detects the sensor current Ipwhich flows in accordance with the air-fuel ratio at each time in amanner similar to the routine of FIG. 6 and applies an applicationvoltage corresponding to the sensor current Ip to the A/F sensor 30. Inthe embodiment, the sensor current Ip is calculated in step 103′ fromthe arithmetic expression;

Ip=(Vc−Va)/R11

(where, R11 is a resistance value of the current detection resistor 15a).

Then, the CPU 21 discriminates whether the sensor current Ip at thattime lies within a range of “−7 mA to 7 mA” or not in step 106. Ip=−7 mAand 7 mA are thresholds used to discriminate whether the air-fuel ratioat that time lies within the first air-fuel ratio zone (A/F=12.8 to 18)or not. When the step 106 is affirmatively discriminated, it denotesthat the air-fuel ratio at that time lies within the first air-fuelratio zone.

When the step 106 is affirmatively discriminated (in case of −7 mA≦Ip≦7mA), the CPU 21 allows the switch circuit 52 to the voltage Ve side instep 111. Consequently, the input voltage Vf of the voltage follower 17becomes the voltage Ve and the Ve value is outputted as an A/F output tothe A/D converter 41 in the engine control ECU 40. The A/F outputdetected by the sensor current detection circuit 15 in this instance isdetected by the sum “R11+R12+R13”, of the resistance values of thecurrent detection resistors 55 a, 55 b, and 15 c.

When the step 106 is negatively discriminated, the CPU 21 discriminateswhether the sensor current Ip at that time is within a range of “15.5 mAor smaller” or “less than −7 mA” in step 112. Ip=15.5 mA is a thresholdused to discriminate whether the air-fuel ratio at that time lies withinthe second air-fuel ratio zone (A/F=18 to 22) or not. When the step 112is affirmatively discriminated, it denotes that the air-fuel ratio atthat time is within the second air-fuel ratio zone.

When the step 112 is affirmatively discriminated (in case of 7mA<Ip≦15.5 mA or Ip<−7 mA), the CPU 21 allows the switch circuit 52 tobe connected to the voltage Vd side in step 113. Consequently, thevoltage Vd serves as the input voltage Vf of the voltage follower 17 andthe Vd value is outputted as an A/F output to the A/D converter 41 inthe engine control ECU 40. The A/F output detected by the sensor currentdetection circuit 15 is detected by the sum “R11+R12” of both of theresistance values of the current detection values 15 a and 15 b.

When the step 112 is negatively discriminated, the CPU 21 allows theswitch circuit 52 to be connected to the voltage Vc side in step 114.Consequently, the voltage Vc serves as the input voltage Vf of thevoltage follower 17 and the Vc value is outputted as an A/F output tothe A/D converter 41 in the engine control ECU 40. In this instance, theA/F output detected by the sensor current detection circuit 15 isdetected by the resistance value “R11” of the current detection resistor15 a.

According to the second embodiment as described above, in a mannersimilar to the first embodiment, the air-fuel ratio can be detected withhigh accuracy while limiting the output voltage of the air-fuel ratiodetecting apparatus to the voltage range which is readable by the A/Dconverter 41. Therefore, effects such that the detection accuracy of theair-fuel ratio can be improved even when a wide air-fuel ratio detectionzone is required can be obtained. As a result, also in the air-fuelratio control system in which both of the stoichiometric control andlean-burn control are performed, while assuring detection accuracy ofthe air-fuel ratio at the time of lean-burn control, the detectionaccuracy of air-fuel ratio near the stoichiometric ratio can beimproved.

Especially, in this embodiment, since the sensor current detectioncircuit 15 is constructed by the three current detection resistors 15 a,15 b, and 15 c, the air-fuel ratio detection with higher precision canbe realized as compared with the first embodiment in which it isconstructed by two current detection resistors. Since the resistancevalue is switched at the air-fuel ratio point (for instance, targetair-fuel ratio) where the detection accuracy is required also in theembodiment, the air-fuel ratio detection as required can be realized.

In connection, four or more current detection resistors of the sensorcurrent detection circuit may be provided and the air-fuel ratio zonefor switching the resistance value can be further divided. In this case,the detection accuracy of the air-fuel ratio can be further improved.

(Third embodiment)

The third embodiment of the invention will be described with referenceto FIG. 13. FIG. 13 is a circuit diagram showing the outline of anair-fuel ratio detecting apparatus in the embodiment. The constructionof the apparatus is basically similar to that of FIG. 1 of the firstembodiment, so that only different points will be described hereinbelow.

According to the configuration of FIG. 13, a sensor current detectioncircuit 15 and a switch circuit 18 are serially connected between theoutput terminal of the amplifier 14 a and the terminal 25 of the A/Fsensor 30. The sensor current detection circuit 15 has two currentdetection resistors 15 a and 15 b which are connected in parallel. Thepoint F between the output terminal of the amplifier 14 a and the switchcircuit 18 is connected to the non-inversion input terminal of thevoltage follower 17 which receives the sensor current Ip as a voltagesignal. The voltage Vf at the point F in the diagram is also received bythe A/D converter 22.

In this case, when the switch circuit 18 is turned to the voltage Vcside as shown in the diagram, the voltage Vc serves as the voltage Vf ofthe input terminal of the voltage follower 17. That is, the sensorcurrent Ip according to the air-fuel ratio is detected by the currentdetection resistor 15 a and the voltage Vc corresponding to Ip issupplied to the voltage follower 17 via the point F in the diagram. Whenthe switch circuit 18 is switched from the position shown in the diagramto the voltage Vd side, the voltage Vd serves as the voltage Vf of theinput terminal of the voltage follower 17. That is, the sensor currentIp is detected by the current detection resistor 15 b. The voltage Vdcorresponding to Ip is supplied to the voltage follower 17 via the pointF in the diagram.

The switching operation of the switch circuit 52 will be described byshowing actual specific values. Methods of detecting the air-fuel ratiowill be described with respect to the following two zones.

the zone near the stoichiometric ratio (A/F=12.8 to 18) in the dynamicrange

air-fuel ratio zones except for the zone near the stoichiometric ratio(A/F=12 to 12.8, 18 to 25)

In the embodiment, the reference voltage Va is set to “2.5V”, the sensorcurrent Ip when A/F=18 is set to “7 mA”, and the sensor current Ip whenA/F=25 is set to “22 mA” (V-I characteristic of FIG. 3). A resistancevalue R21 of the current detection resistor 15 a is set to “113 Ω” and aresistance value R22 of the current detection resistor 15 b is set to“357 Ω”.

First, in the zone near the stoichiometric ratio (A/F=12.8 to 18), theair-fuel ratio at which the voltage Vc or the voltage vd is maximum isA/F=18 and the voltages Vc and vd when A/F=18 are as follows.

Vc=3.291V

Vd=4.999V

The voltage Vc is obtained by adding the reference voltage Va to theproduct of the sensor current Ip and the resistance value R21 of thecurrent detection resistor 15 a (Vc=Ip·R21+Va). The voltage Vd isobtained by adding the reference voltage Va to the product of the sensorcurrent Ip and the resistance value R22 of the current detectionresistor 15 b (Vd=Ip·R22+Va).

Since both of the voltages Vc and Vd are within a voltage range (0 to5V) which can be dealt by the A/D converter 41 in the engine control ECU40, both of the values can be read by the A/D converter 41. As describedabove, in order to assure the detection accuracy, however, it isdesirable to set the range of voltage value per unit A/F as large aspossible. In this case, since the voltage Vd has the voltage value perunit A/F larger than the voltage Vc as described, it can be the that thevoltage Vd has higher detection accuracy. There is a similar tendencyfor any A/F value if it is within the zone near the stoichiometric ratio(A/F=12.8 to 18). That is, the Vd value is used as the input voltage Vfof the voltage follower 17 in the zone near the stoichiometric ratio,the detection accuracy of the air-fuel ratio can be assured (FIGS. 4 and5). It is sufficient that the value of the current detection resistor is“R22=357 Ω (value of the current detection resistor 15 b).

On the other hand, the air-fuel ratio at which the voltage Vc or thevoltage vd is maximum is A/F=25 in the air-fuel ratio zones (A/F=12 to12.8, 18 to 25) other than the zone near the stoichiometric ratio. Thevoltages Vc and Vd when A/F=25 are as follows.

Vc=4.986V

Vd=10.354V

In this case, since the input voltage range of the A/D converter 41 is 0to 5V, although the voltage Vc can be read, the voltage Vd cannot beread. In the air-fuel ratio zones (A/F=12 to 12.8, 18 to 25), therefore,the Vc value is used as the input voltage Vf of the voltage follower 17.That is, it is sufficient that the value of the current detectionresistor is “R21=113 Ω” (value of the current detection resistor 15 a).

In the air-fuel ratio detecting apparatus having the aboveconfiguration, an air-fuel ratio detecting process is performedbasically according to the routine of FIG. 6 of the first embodiment(since the main construction is based on FIG. 6, a diagram is omittedhere). When a point different from FIG. 6 is mentioned, the followingequation is used to calculate the sensor current Ip (step 103 in FIG. 6)in the embodiment.

Ip=(Vf−Va)/R21 or

Ip=(Vf−Va)/R22

That is, the sensor current Ip is calculated by using the potentialdifference between the voltage Vf and the voltage Va and the resistancevalue R21 (or R22) of the current detection resistor 15 a (or 15 b).

In the embodiment described above in detail, different from the firstand second embodiments, the current detection resistors 15 a and 15 b inthe sensor current detection circuit 15 are connected in parallel. In amanner similar to the above-mentioned embodiments, however, an effectsuch that the detection accuracy of the air-fuel ratio is improved evenwhen a wide air-fuel ratio detection range is required is obtained. As aresult, also in the air-fuel ratio control system in which both of thestoichiometric control and the lean-burn control are performed, thedetection accuracy of the air-fuel ratio near the stoichiometric ratiocan be improved while assuring the detection accuracy of the air-fuelratio at the time of the lean burn control.

As another example of the third embodiment, it can be considered toembody the invention as follows. The sensor current detection circuit 15has three or more current detection resistors which have differentresistance values and are connected in parallel and the air-fuel ratiozone for switching the resistance value of the current detectionresistor is further divided. In this case, the detection accuracy of theair-fuel ratio can be further improved.

(Fourth embodiment)

The fourth embodiment of the invention will be described with referenceto FIG. 12. FIG. 12 is a circuit diagram showing the outline of anair-fuel ratio detecting apparatus in the embodiment. Since theconstruction of the apparatus is basically similar to that of FIG. 1 ofthe first embodiment, only different points will be describedhereinbelow.

In the air-fuel ratio detecting apparatuses in the first to thirdembodiments, the switch circuit is arranged in the bias control circuit10 and the CPU 21 variably sets the resistance value of the currentdetection resistor by switching the switch circuit in accordance withthe sensor current Ip at each time. The detection accuracy of theair-fuel ratio is assured by the switching operation. On the contrary,in the apparatus of the present embodiment, the value of the currentflowing in the A/F sensor 30 is outputted as a plurality of detectionsignals (A/F values) having different voltage levels. One of theplurality of detection signals to be used is selected in the enginecontrol ECU 40 in accordance with the sensor current Ip.

In FIG. 14, the sensor current detection circuit 15 is provided betweenthe output terminal of the amplifier 14 a and the terminal 25 of the A/Fsensor 30. Input terminals of voltage followers 71 and 72 are connectedto the points C and D, respectively, between the output terminal of theamplifier 14 a and the terminal 25 of the A/F sensor 30. The voltages Vcand Vd at the points C and D are applied to the voltage followers 71 and72. The A/D converter 41 in the engine control ECU 40 is connected tothe output terminals of the voltage followers 71 and 72 via clampcircuits 73 and 74 as voltage guarding means. The clamp circuit 73comprises a pair of diodes 73 a and 73 b connected between the constantvoltage Vcc and the ground. Similarly, the clamp circuit 74 comprises apair of diodes 74 a and 74 b connected between the constant voltage Vccand the ground. These clamp circuits 73 and 74 guard the outputs of thevoltage followers 71 and 72 at the constant voltage Vcc.

According to the above configuration, different from the embodimentsdescribed, the switching operation of the switch circuit is unnecessary.The voltages Vc and Vd are applied to the engine control ECU 40 via thevoltage followers 71 and 72. That is, the sensor current Ip flowing inthe current detection resistors 15 a and 15 b is outputted as two A/Fsignals (voltages Vc and Vd). In a manner similar to the firstembodiment, the resistance value R1 of the current detection resistor 15a is “113 W” and the resistance value R2 of the current detectionresistor 15 b is “244 Ω”.

In this case, the engine control ECU 40 selects one of the two A/Fsignals in accordance with the air-fuel ratio (sensor current Ip) ateach time. Specifically, when the sensor current Ip is “−7 mA to 7 mA”,that is, when the air-fuel ratio at that time is within the zone nearthe stoichiometric ratio (A/F=12.8 to 18), the A/F value can be detectedby the voltage Vd. When the sensor current Ip is “out of the range from−7 mA to 7 mA”, that is, when the air-fuel ratio at that time is withinthe air-fuel ratio zones (A/F=12 to 12.8, 18 to 25) other than the zonenear the stoichiometric ratio, the A/F value can be detected by thevoltage Vc (characteristic diagram of FIG. 4 for details).

According to the fourth embodiment, in a manner similar to the foregoingembodiments, even when a wide air-fuel ratio detection range isrequired, the detection accuracy of the air-fuel ratio can be improvedand the object of the invention can be achieved. In this case, the morethe sensor current Ip is away from the stoichiometric ratio, a detectionsignal of an electric resistor having a low resistance value may beselected among the plurality of detection signals (Vc, Vd) (similaroperation is performed when three or more current detection resistorsare used).

In the embodiment, the clamp circuits 73 and 74 for regulating theoutput voltages of the voltage followers 71 and 72 so as to be in apredetermined voltage range which can be read by the A/D converter 41,that is, in the voltage range of 0 to 5V are provided. In this case, theoutput voltages of the voltage followers 71 and 72 become voltagesignals which can be always read by the A/D converter 41, so that it canbe prevented that an excess voltage is applied to the A/D converter 41.The invention can be also embodied by omitting the clamp circuits 71 and72.

(Fifth Embodiment)

Next, the fifth embodiment will be described with reference to FIGS. 15to 17. In this embodiment, when a resistance value of the sensor currentdetection circuit in the air-fuel ratio detecting apparatus is switched,the switching information is transmitted to an engine control ECU 40.Thus, even when the output voltages become the same against differentair-fuel ratios, the engine control ECU 40 detects the air-fuel ratioaccurately.

FIG. 15 is a circuit diagram showing an outline of the air-fuel ratiodetecting apparatus according to the present embodiment.

In FIG. 15, differeently from FIG. 1, the engine ECU 40 is provided witha digital port 43 which is connected to the CPU 21 through a signal line44. The digital port 43 is held at either “0” or “1” in accordance withthe range signal transmitted from the CPU 21.

FIG. 16 is a flowchart which shows a part of the air-fuel ratiodetection routine executed by the CPU 21 and is a partial modificationof the process shown in FIG. 6. In FIG. 16, the CPU 21 drives at steps106 to 108 the switch circuit 18 in accordance with the sensor currentIp. That is, in the case of Ip=−7 mA to 7 mA, the CPU 21 drives theswitch circuit 18 to the voltage Vd side (step 107) and, in the case ofIp<−7 mA or Ip>7 mA, drives the switch circuit 18 to the voltage Vc side(step 108).

When the switch circuit 18 is connected to the voltage Vd side, the CPU21 clears the range signal to “0” at step 201. When the switch circuit18 is connected to the voltage Vc side, the CPU 21 sets the range signalto “1” at step 202.

The engine control ECU 40 discriminates whether the range signal is “0”or “1” from the signal condition at the digital port 43. If the rangesignal is 0, the ECU 40 discriminates that the air-fuel ratio isdetected in the range near the stoichiometric air-fuel ratio (A/F=12.8to 18). If the range signal is 1, the ECU 40 discriminates that theair-fuel ratio is detected in the range other than the range near thestoichiometric air-fuel ratio.

FIG. 17 is a time chart showing in more detail the operation of theembodiment. In this figure, transitions of the sensor current Ip, A/Foutput voltage and range signal which occur when the air-fuel ratiochanges from a rich value (for example, A/F=13) to a lean value (forexample, A/F=25). It is assumed that the air-fuel ratio reaches thestoichiometric ratio at time t1 and reaches “18” at time t2.

Before time t2, the range signal is maintained at “0” because of Ip=−7mA to 7 mA. As the sensor current Ip attains at time t2 “7 mA” whichcorresponds to A/F=18, the range signal is switched to “1”. The enginecontrol ECU 40 detects from the range signal that the switch circuit 18is driven. Thus, even when the sensor output voltages are the same, twoA/F values in the range near the stoichiometric ratio and in the otherrange.

According to the fifth embodiment, the engine control ECU 40 is enabledto determine accurately the air-fuel ratio (A/F value). As a result, theengine control ECU 40, thus determining the air-fuel ratio accurately,can perform a highly accurate air-fuel ratio control in response to thedetermined air-fuel ratio.

As a modification of the fifth embodiment, more than three currentdetection resistors may be switched over. In the case of switching threecurrent detection resistors, for example, even when the sensor outputvoltages are the same, the engine control ECU 40 detects three A/Fvalues for each air-fuel ratio range in response to the range signal.

(Sixth Embodiment)

Next, the sixth embodiment is described. In this embodiment, the enginecontrol ECU 40 outputs a switching command to the switch circuit 18 andthe CPU 21 performs switching operation of the switch circuit 18 inresponse to the command.

FIG. 18 is a flowchart showing a timer interrupt routine executed by theengine control ECU 40. In FIG. 18, the engine control ECU 40discriminates first at step 301 whether the lean control condition holdsor not. The lean control condition includes, for examle:

the A/F sensor 30 is activated; and

the engine is in normal operation.

If the lean control condition holds, the engine control ECU 40 proceedsto step 302 to set the range signal to “1”. If the lean controlcondition does not hold, the engine control ECU 40 proceeds to step 303to set the range signal to “0”.

The CPU 21 performs switching of the switch circuit 18 in response tothe range signal set by the engine control ECU 40. That is, in theair-fuel ratio detection routine shown in FIG. 19, the CPU 21discriminates at step 401 whether the range signal is “0” or not. If therange signal=0, the CPU 21 connects at step 107 the switch circuit 18 tothe voltage Vd side. At this time, the air-fuel ratio near thestoichiometric ratio is detected. If the range signal=1, the CPU 21connects at step 108 the switch circuit 18 to the voltage Vc side. Atthis time, the air-fuel ratio near the lean burn zone is detected. It ispossible also in this embodiment to use more than three currentdetection resistors.

(Seventh Embodiment)

In this embodiment, the resistance value of the current detectionresistor is set variably when sensor deteriortion is to be detected froma result of detecting atmospheric gas air-fuel ratio in the midst of theair-fuel ratio control. At the time of atmospheric gas air-fuel ratiodetection, the air-fuel ratio becomes extreme lean and the sensorcurrent IP becomes for example 136 mA”. The resistance value of thecurrent detection resistor is changed to “69 Ω” so that the air-fuelratio may be detected within the signal processing range (0V to 5V) ofthe A/D converter 41 in the same manner as in the above embodiments. Bysetting the resistance value to 69 Ω, the input voltage Vf of thevoltage follower 17 in FIG. 1, for instance, becomes:

Vf=36 mA·69 Ω+2.5V=4.984V

which is readable by the A/D converter 41. To be more specific, theengine control ECU 40 executes the processing in FIG. 20.

In FIG. 20, the engine control ECU 40 discriminates at step 501 whetherit is in the fuel cut-off operation (F/C) at present or not. If NO, step502 commands to CPU 21 an air-fuel ratio detection by the resistancevalue for the air-fuel ratio control. At this time, the CPU 21 performsswitching of the switch circuit in response to the command from theengine control ECU 40.

If the discrimination at step 501 is YES, the engine contorol ECU 40proceeds to step 503 to command to the CPU 21 an air-fuel ratiodetection by the resistance value for the atmospheric gas air-fuel ratiodetection. At this time, the CPU 21 performs switching of the switchcircuit in response to the command from the engine control ECU 40. Then,the engine control ECU 40 detects the atmospheric gas air-fuel ratio atstep 504 and discriminates deterioration of the A/F sensor 30 from thedetected atmospheric air-fuel ratio at step 505. As an example, thesensor current Ip detected at step 504 is compared with a sensor current(predetermined threshold Ith) known at the time of atmospheric gasair-fuel ratio detection. If the Ip value and the Ith differ greately,the sensor deterioration may be discriminated. If Ip<Ith, the sensordeterioration is discriminated as clogging in the electrodes of the A/Fsensor 30 or small holes 31 a of the cover 31 or as peeling-off of theelectrodes.

As described above, the present invention may be applied even at thetime of detecting the atmospheric gas air-fuel ratio detection. That is,the air-fuel ratio (sensor current Ip) can be detected accurately ateither time of the air-fuel ratio control or the atmospheric gasair-fuel ratio control. Further, even at the time of detecting theatmospheric gas air-fuel ratio, no undesired influence will affect theair-fuel ratio feedback control operation.

As a current detection resistor of the sensor current detection circuit,a variable resistor whose resistance value can be optionally changed isused. In this case, as shown by a characteristic line L1 in FIG. 21, itis sufficient to change the resistance (Ω) of the current detectionresistor in accordance with the sensor current Ip. In FIG. 21, acharacteristic line L2 shown by a broken line is the same as thecharacteristic line shown in FIG. 11. When a plurality of currentdetection resistors are switched, it is sufficient to set the switchingpoint on the characteristic line L1. Therefore, switching points ofresistance values may be set arbitrarily on the line Ll in FIG. 21 otherthan the above-described resistance value switching points such asA/F=18 (Ip=7 mA) and A/F=22 (Ip=15.5 mA).

Although the reference voltage Va generated by the reference voltagecircuit 11 is set to “2.5V” in the foregoing embodiments, the value maybe changed. For example, in case of setting the reference voltage Va toa value smaller than “2.5V”, the characteristic lines shown in FIGS. 5,9, and 21 are shifted to the right side in the diagrams.

In recent years, an in-cylinder direct injection type engine in whichfuel is injected directly into a cylinder (combustion chamber) of theengine is implemented. In the direct injection type engine, an air-fuelratio control near an extreme lean zone (A/F=around 40) can be realized.In an air-fuel ratio control system in which the lean-burn control inthe extreme lean zone is used, the dynamic range (air-fuel ratiodetection range) is set to A/F=12 to 40 and the resistance value of thecurrent detection resistor is variably set in the dynamic range. In thelean-burn control in the extreme lean zone, the target air-fuel ratio isset to, for example, A/F=around 37.

Specifically, when the reference voltage (FIG. 1) is set to “2.5V” andthe sensor current Ip when A/F=40 is “28 mA”, it is sufficient to setthe resistance value of the current detection resistor to “89 Ω”. Thatis, it is sufficient to use, for example, the voltage Vf of thenon-inversion input terminal of the voltage follower 17 shown in FIG. 1as a voltage value to be detected by the current detection resistorhaving the resistance value=89 Ω. In this case, the input voltage Vf ofthe voltage follower 17 is obtained by

Vf=28 mA·89 Ω+2.5V=4.992V,

which is a voltage value that can be read by the A/D converter 41. In amanner similar to, for instance, the first embodiment, therefore, whenthe air-fuel ratio point at which the current detection resistors areswitched is set to, for example, A/F=12.8, 18 and in addition, A/F=25,it is sufficient that

the resistance value of the current detection resistor is set to “357 Ω”when A/F=12.8 to 18,

the resistance value of the current detection resistor is set to “113 Ω”when A/F=12 to 12.8, 18 to 25, and

the resistance value of the current detection resistor is set to “89 Ω”when A/F=25 to 40.

The control in the extreme lean zone as mentioned above can be alsoapplied to embodiments such as the fourth embodiment in which aplurality of voltage signals for respective air-fuel ratio zones areproduced to the engine control ECU 40.

Further, although the switching of the switch circuit 18 is performed bythe CPU 21 in the foregoing embodiments, it may be performed by the CPU42 in the engine control ECU 40 in correspondence with required A/Faccuracy.

Although the voltage value adjustment is performed based on the outputvoltage produced from the voltage follower 17 when the constant current(desired sensor current corresponding to the predetermined air-fuelratio) is supplied by the constant current source 101 in FIG. 8 in thefirst embodiment, the output voltage may be adjusted by connecting theA/F sensor 30 to be used actually in place of the constant currentsource 101 in FIG. 8. In this instance, variations (variations among thedevices) are reduced and the detection accuracy can be enhanced more.

Still further, the above output voltage adjustment (FIGS. 7 and 8) maybe applied not only to the apparatus which sets variably the resistancevalue of the sensor current detection circuit but also to other air-fuelratio detecting apparatus.

That is, it may be applied to air-fuel ratio detecting apparatuses whichconverts a sensor current to a voltage value, as long as the outputvoltage is adjusted by trimming voltage dividing resistors which producea reference voltage. According to this output voltage adjustment, ahighly accurate air-fuel ratio control apparatus can be provided evenwhen a lean-burn system or direct injection engine is used or a morestrict exhaust regulation is introduced in the furture.

Although the construction in which the voltage applied to the A/F sensor30 is variably controlled by the bias control circuit 10 is used in theforegoing embodiments, the voltage applied to the A/F sensor 30 may befixed. For example, in the construction shown in FIG. 1, the CPU 21, theA/D converter 22 and the D/A converter 23 are omitted and the switchingoperation of the switch circuit 18 is controlled by the engine controlECU 40.

Although the sensor current detection circuit is provided only on theterminal 25 side which is connected to the atmosphere side electrodelayer 37 in the A/F sensor 30, this arrangement may be changed. Forexample, the sensor current detection circuit may be provided on theterminal 26 side connected to the outer atmosphere-gas side electrodelayer 36 in the A/F sensor 30 or the sensor current detection circuitsmay be provided on both of the terminals 25 and 26. In short, it issufficient as long as the sensor current detection circuit is providedin an electric path through which the sensor current Ip flows and theA/F signals at different voltage levels are obtained by the currentdetection resistor of the sensor current detection circuit.

Although the invention is embodied in the one-cell type limit currenttype air-fuel ratio sensor as the above embodiments, it may be changed.For example, the invention may be embodied in a two-cell type air-fuelratio sensor. In the two-cell type air-fuel sensor, the air-fuel ratiois detected in accordance with a pumping current supplied to the sensor.Further, the invention may be embodied to a stack-type A/F sensor inplace of the cup-shaped A/F sensor.

The present invention may be applied to apparatuses other than theair-fuel ratio detecting apparatus which uses the air-fuel ratio sensor.That is, the invention may be applied to a gas concentration detectingapparatus which uses a gas concentration sensor capable of detectingconcentration of gas components such as NOx, HC, CO or the like.

What is claimed is:
 1. A manufacturing method for manufacturing acurrent-generating type gas concentration detector circuit having itsoutput coupled to an analog-to-digital converter working in apredetermined input signal range, said method of manufacturingcomprising: temporarily substituting into the circuit a constant currentsource for the gas concentration detector; permanently adjusting aresistance of a reference voltage divider which provides a referencevoltage normally applied to the gas detector to have a predeterminedreference valve corresponding to the constant current source andmagnitude; and removing said constant current source from the circuitand substituting the gas concentration detector back into said circuitfor normal use thereafter in detecting gas concentrations.
 2. Amanufacturing method for manufacturing a gas concentration detectingapparatus including a gas concentration sensor for outputting a currentsignal corresponding to a gas concentration to be detected when areference voltage is applied and which converts a current signaloutputted from the sensor into a voltage signal output after A/Dconversion to a signal processor which uses an A/D converted voltagesignal in a predetermined voltage range, the method comprising the stepsof: connecting a constant current source in place of said gasconcentration sensor; monitoring the voltage signal; adjusting thevoltage signal by trimming at least one of a plurality of voltagedividing resistors which produce a reference voltage applied to the gasconcentration sensor; and removing said constant current source from thecircuit and substituting the gas concentration sensor into saidapparatus for normal use thereafter in detecting gas concentrations. 3.A manufacturing method as in claim 2, further comprising the steps of:converting the current signal of the gas concentration sensor by aresistor connected between the constant current source and anoperational amplifier to which an output of the voltage dividingresistors is applied.
 4. A manufacturing method as in claim 3, wherein aresistance of the at least one of the voltage dividing resistors isadjusted to fix the monitored voltage singal to a fixed value which isinvariable with the current signal.
 5. A manufacturing method for anengine condition detecting apparatus which includes a sensor foroutputting a current signal corresponding to an engine condition to bedetected when a bias voltage is applied, and a bias control circuitwhich applies the bias voltage to the sensor and converts the currentsignal outputted from the sensor into an output voltage signal, themethod comprising the steps of: connecting a constant current source tothe bias control circuit in place of the gas concentration sensor tocause the bias control circuit to produce the output voltage signal inresponse to a constant current outputted from the constant currentsource; monitoring the output voltage signal produced from the biascontrol circuit; trimming a part of the bias control circuit until themonitored output voltage signal equals a fixed voltage signalpredetermined in correspondence with the constant current; and removingsaid constant current source from the circuit and substituting the gasconcentration sensor into said apparatus for normal use thereafter indetecting gas concentrations.
 6. A manufacturing method as in claim 5,wherein: the bias control circuit includes a reference voltage sourcefor supplying a reference voltage, and a resistor connected between thereference voltage source and the constant current source to produce theoutput voltage signal; and the trimming step trims the reference voltagesource.
 7. A manufacturing method as in claim 6, wherein: the sensor isa gas concentration responsive type for detecting a gas concentration ofan engine; the reference voltage source includes a plurality ofresistors for supplying the reference voltage; and the trimming steptrims a resistance of at least one of the resistors of the referencevoltage source.
 8. A manufacturing method as in claim 5, wherein thecurrent signal is converted into the output voltage by a resistor whichis connected to a source of the fixed voltage through an operationalamplifier.
 9. A manufacturing method as in claim 8, wherein the biascontrol circuit includes a resistor, and a resistance of the resistor isvaried until the monitored output voltage signal equals the fixedvoltage signal.